Wireless Networks L ecture 19: MIMO Peter Steenkiste CS and ECE, - - PDF document

Page 1

Peter A. Steenkiste

1

Wireless Networks Lecture 19: MIMO

Peter Steenkiste CS and ECE, Carnegie Mellon University Peking University, Summer 2016

Peter A. Steenkiste

2

Increasing Capacity: MIMO

 Refresher: spatial diversity  Multiple-In Multiple-Out basics  MIMO in 802.11: » Single user MIMO: 802.11n » Multi user MIMO: 802.11ac

Page 2

Peter A. Steenkiste

3

How Do We Increase Throughput in Wireless?

 Wired world:

Pull more wires!

 Wireless world:

How about if we could do the same thing and simply use more antennas?

Peter A. Steenkiste

4

MIMO Multiple In Multiple Out

 N x M subchannels that can be used to send

multiple data streams simultaneously

 Fading on channels is largely independent » Assuming antennas are separate ½ wavelength or more  Combines ideas from spatial and time

diversity, e.g. 1 x N and N x 1

 Very effective if there is no direct line of sight » Subchannels become more independent

N transmit antennas M receive antennas

Page 3

Peter A. Steenkiste

5

Why So Exciting?

Method SISO Diversity (1xN or Nx1) Diversity (NxN) Multiplexing Capacity B log2(1 + ) B log2(1 + ) B log2(1 + ) NB log2(1 + )

Peter A. Steenkiste

6

Spatial Diversity

 Use multiple antennas that pick up the signal

in slightly different locations

» Channels uncorrelated with sufficient antenna separation  Receiver diversity:  Receiver can pick strongest signal: y1 or y2  Or combines the signals: multiply y with the

complex conjugate h* of the channel vector h

» Can learn h based on training data

h1 h2 x y1 y2 y = h * x + n y y =h* * (h * x + n) i x H x PR = o

Page 4

Peter A. Steenkiste

7

Other Diversity Options

 Transmit diversity:  Combined:

h1 h2 y x1 x2 x h11 h22 y1 x1 x2 x y y2 h12 h21 i x PT x H = o i x PT x H x PR = o

Peter A. Steenkiste

10

MIMO How Does it Work?

 Transmit and receive multiple data streams  Coordinate the processing at the transmitter

and receiver to overcome channel impairments

» Maximize throughput or minimize interference

I x PT x H x PR = O

 Combines previous techniques T R

Channel Matrix Precoding Combining

Page 5

Peter A. Steenkiste

11

Mechanisms Supported by MIMO

Peter A. Steenkiste

12

An Example of Space Coding

Page 6

Peter A. Steenkiste

14

Direct-Mapped NxM MIMO

 How do we pick PR ? “Inverse” of H: H-1 » Equivalent of nulling the interfering possible (zero forcing) » Only possible if the paths are completely independent  Noise amplification is a concern if H is non-

invertible – its determinant will be small

» Minimum Mean Square Error detector balances two effects

Effect of transmission R = H * C + N

M MxN N M

Decoding O = PR * R C = I

D DxM M N N

Results O = PR * H * I + PR * N

Peter A. Steenkiste

15

Precoded NxM MIMO

 How do we pick PR and PT ?  Singular value decomposition of H = U * S * V » U and V are unitary matrices – UH*U = VH*V = I » S is diagonal matrix

Effect of transmission R = H * C + N

M MxN N M

Coding/decoding O = PR * R C = PT * I

D DxM M N NxD D

Results O = PR * H * PT * I + PR * N

Page 7

Peter A. Steenkiste

17

MIMO Discussion

 Need channel matrix H: use training with

known signal

 So far we have ignored multi-path » Each channel is multiple paths with different properties » Becomes even messier!  MIMO is used in 802.11n » Can use two adjacent non-overlapping “WiFi channels” » Raises lots of compatibility issues » Potential throughputs of 100s of Mbps  Focus is on maximizing throughput between

two nodes

» Is this always the right goal?

Peter A. Steenkiste

18

802.11n Overview

 802.11n extends 802.11 for MIMO » Supports up to 4x4 MIMO » Preamble that includes high throughput training field  Standardization was completed in Oct 2009, but,

early products have long been available

» WiFi alliance started certification based on the draft standard in mid-2007  Supported in both the 2.4 and 5 GHz bands » Goal: typical indoor rates of 100-200 Mbps; max 600 Mbps  Use either 1 or 2 non-overlapping channels » Uses either 20 or 40 MHz » 40 MHz can create interoperability problems  Supports frame aggregation to amortize

• verheads over multiple frames

» Optimized version of 802.11e

Page 8

Peter A. Steenkiste

19

802.11n Backwards Compatibility

 802.11n can create interoperability problems for

existing 802.11 devices (abg)

» 802.11n does not sense their presence » Legacy devices end up deferring and dropping in rate  Mixes Mode Format protection embeds an n

frame in a g or a frame

» Preamble is structured so legacy systems can decode header, but MIMO can achieve higher speed (training, cod/mod info) » Works only for 20 MHz 802.11n use » Only deals with interoperability with a and g – still need CTS protection for b  For 40 MHz 802.11n, we need CTS protection on

both the 20 MHz channels – similar to g vs. b

» Can also use RTS/CTS (at legacy rates) » Amortize over multiple transmissions

Peter A. Steenkiste

20

MIMO in a Network Context

N transmit antennas M receive antennas N transmit antennas M receive antennas

• M receivers

How is this Different?

Page 9

Peter A. Steenkiste

21

Multi-User MIMO Discussion

 Math is similar to MIMO, except for the

receiver processing (PR)

» Receivers do not have access to the signals received by antennas on other nodes » Limits their ability to cancel interference and extract a useful data stream » Closer to transmit MRC  MU-MIMO versus MIMO is really a tradeoff

between TDMA and use of space diversity

» Sequential short packets versus parallel long packets  Why not used in 802.11?

Peter A. Steenkiste

22

Multi-User MIMO Up versus Down Link

 Uplink: Multiple Access Channel (MAC) » Multiple clients transmit simultaneously to a single base station » Requires coordination among clients on packet transmission – hard problem because very fine-grained  Downlink: Broadcast Channel (BC) » Base station transmit separate data streams to multiple independent users » Easier to do: closer to traditional models of having each client receive a packet from the base station independently

Page 10

Peter A. Steenkiste

23

802.11ac Multi-user MIMO

 Extends beyond 802.11n » MIMO: up to 8 x 8 channels (vs. 4 x 4) » More bandwidth: up to 160 MHz by bonding up to 8 channels (vs. 40 MHz) » More aggressive signal coding: up to 256 QAM (vs. 64 QAM); both use 5/6 coding rate (data vs. total bits) » Uses RTS-CTS for clear channel assessment » Multi-gigabit rates (depends on configuration)  Support for multi-user MIMO on the downlink » Can support different frames to multiple clients at the same time » Especially useful for smaller devices, e.g., smartphones » Besides beam forming to target signal to device, requires also nulling to limit interference

Peter A. Steenkiste

24

802.11ad 60 GHz WiFi

 Uses a new physical layer definition

specifically for 60 GHz band

» Very different signal propagation properties » Does not penetrate walls, but does work with reflections » Shorter distances » Small antennas and good beamforming properties  Defined up to 7 Gbps  Has been used for point-point links for a while » APs now available » Combined with other 802.11 versions » 802.11ad only available for short distances